Many glass articles and articles containing glass are required to present a glass surface having excellent chemical and mechanical durability. Glass has a general tendency that a glass surface machined as by cutting and polishing is markedly reduced in chemical and mechanical durability as compared with the base glass. This tendency becomes more outstanding with glasses having lower working temperatures Tw. It is to be noted that the working temperature Tw of glass used herein is a temperature at which the glass exhibits a viscosity of 10.sup.4 poise. It is thus desired to reinforce a machined surface of glass.
Several methods for reinforcing a glass surface are known in the art including ion exchange, ion implantation, and surface quenching. It is also possible to control a glass composition so as to reinforce the entire glass. However, control of glass composition for improved chemical and mechanical durability is somewhat limitative because the glass must meet the thermal properties required for each type of article. The composition control can improve chemical and mechanical durability to a certain, but limited extent, failing to provide sufficient durability.
As opposed to the reinforcement of an entire glass mass, the reinforcement of only a glass surface leaves an increased degree of freedom to the feasibility of glass composition because in most techniques, the physical properties of the base glass are negligible. However, conventional techniques like ion exchange, ion implantation, and surface quenching are applicable to only limited objects.
The ion exchange technique is useful to glasses containing a substantial amount of alkali metals such as sodium, and often applied to glasses having a relatively high working temperature Tw. However, the ion exchange is ineffective to glasses having reduced contents of alkali metals, especially alkali-free glasses and glasses having low working temperatures cannot withstand the ion exchange process. The glasses to which the ion exchange technique can be applied are only those having a composition meeting the necessary conditions.
The ion implantation technique is a less productive technique requiring a large size of equipment. When it is applied to articles partially containing glass, for example, magnetic heads, ions are implanted to undesired portions other than the glass surface, with possible deterioration of the articles.
The surface quenching technique is difficult to apply to glass having a low working temperature. If glass joined or bonded to another material is quenched, the differential thermal behavior would create stresses to induce cracks in the glass.
The prior art method for reinforcing only a glass surface is predominantly to enhance the mechanical durability of glass. Even if the treatment is successfully done under various limitations, the result is yet insufficient with respect to chemical durability.
The prior art methods are least successful in surface reinforcement of glass having a low melting point or working temperature, for example, high lead glass.
Magnetic heads are generally constructed by mating core halves with welding glass into a core block and joining the core block to a slider with sealing glass. Welding or sealing glass forming the seal, weld or joint between magnetic head components is required to have enough chemical durability to maintain the reliability and performance of the magnetic head. In the case of composite type magnetic heads, for example, the sealing glass which is exposed at the sliding or front surface is required to have high chemical durability at the surface. If the sealing glass exposed at the sliding surface of a magnetic head is degraded or deteriorated or accompanied by formation of foreign matter at the surface due to weathering, corrosion or the like, the magnetic head loses reliability.
A machined surface of glass, for example, a mirror polished surface of glass on the sliding face of a magnetic head is markedly reduced in chemical durability as compared with the base glass. The glass exposed on the magnetic head sliding face is also required to have high mechanical strength and abrasion resistance. The sealing glass used in those areas of the magnetic head sliding face where the glass is exposed is preferably an alkali-free glass having a low working temperature Tw. However, glasses having low working temperatures are generally less durable, especially less chemically durable. Magnetic heads using such a low working temperature sealing glass thus become less reliable.
An attempt was made in the prior art to improve the durability, especially chemical durability of sealing glass by modulating the glass composition. However, modulation of glass composition for improved chemical durability is somewhat limitative because the glass must also meet the thermal properties required as the sealing glass. The composition modulation can improve chemical durability to a certain, but limited extent, failing to provide sufficient chemical durability.
In addition to the reinforcement of an entire glass mass by composition control, it is also attempted to reinforce only a surface of glass. As mentioned above, the surface reinforcement by composition control has a certain limit because the physical properties of an entire glass must be taken into account. In turn, the reinforcement of only a glass surface can be done by some techniques without paying much attention to the physical properties of an entire glass, leaving an increased degree of freedom to the feasibility of glass composition.
Known techniques for glass surface reinforcement include ion exchange, ion implantation, and surface quenching. However, these techniques are almost impossible to apply to magnetic heads where the glass to be surface reinforced is exposed at the sliding face.
The ion exchange technique is useful to glasses containing substantial amounts of alkali metals such as sodium. However, the ion exchange is ineffective to the sealing glass which favors a less alkali metal content. Low working temperature glass for magnetic head sealing cannot withstand the ion exchange.
The ion implantation technique is a less productive technique requiring a large size of equipment. When it is applied to magnetic heads, ions are implanted to undesired portions other than the sealing glass surface, for example, ferrite core block sliding areas with the likelihood of degrading magnetic properties.
The surface quenching technique is difficult to apply to glass having a low working temperature (Tw 600.degree. C or lower) such as a high lead content, low-melting point glass for magnetic head sealing. If the sealing glass joined or bonded to another material in a magnetic head is quenched, the differential thermal behavior would create stresses to induce cracks in the glass.